Common Knowledge – Roll-Enforced Metagaming!

Recently, my home game encountered a mind flayer. It’s a new campaign, and so far everyone is 1st level. Regardless, we all obviously know a mind flayer when we see one. It’d be hard not to!

But our characters are still 1st level. Would they know what an illithid is? And if so, how? I’ve read the mind flayer statblock a million times, but why would my sheltered young noble alchemist have even the slightest clue? Shouldn’t it just be a weird gross purple thing with tentacles?

There’s always been solutions to this, but it would be nice to have a more… specific sort of answer. So, to get to that, click on through below and learn all about common knowledge!


The Concept

At heart, the common knowledge concept is meant as a way to codify what information would or would not be possible for an in-game character to know, especially when that character’s player would know all the details already.

This isn’t a concept that should see much use for new players. After all, if this is your first campaign it’s highly unlikely that you’ll already have read not only the illithid statblock, but also the entire chapter on mind flayers found in Volo’s Guide to Monsters. Like I have. Multiple times.


So how does it work? Firstly, common knowledge is not a skill check. The skill check, whether it be Arcana, History, Religion, or otherwise, is to establish what the character remembers about a topic. The common knowledge system, meanwhile, defines what a character would have been exposed to.

The common knowledge system primarily functions off of culture. Someone who grew up with the gith would have had a multitude of opportunities to learn about illithid. Someone growing up in a human city, meanwhile, would almost certainly have never even heard of them except as passing rumor.

In this way, the system does two things. Firstly it sets a DC for the check by establishing the base likelihood that a character would have encountered this information. Secondly, it then sets the quality of the info by defining what type of knowledge the character would have had access to (written treatises or official instruction vs. rumor, hearsay, and ghost stories).

The basic unit of the common knowledge system is a Tier of Familiarity, of which there are four. Those are detailed below.


Tiers of Familiarity

Common knowledge uses a four-tiered system, ranging from Unknown to Familiar. These tiers determine the DC for skill checks and the quality of information gained from a successful check. In addition, each tier also specifies what occurs on a “Near Fail” – when the roll misses the check DC by 5 or less.

Also included are two optional tiers, one above and one below all the others presented. These are technical tiers just to give framing on what the absolute least and absolute greatest usable tiers have to offer.


A topic that the character would have almost certainly never encountered before. If any rumors did come through, they would have been so infrequent that remembering anything would be a struggle, and any information recalled would likely be incorrect.

  • Check DC: 27
  • Quality: Low
    • Only the most basic of facts could be discovered, such as a creature’s type or a magical effect’s school of magic.
  • Near Fail: N/A
    • If a character fails by any amount, they know nothing on the topic. The character is unable to state whether or not they even recognize the topic.

A topic that the character could have easily been encountered previously, but is so tainted by superstition and general narrative corruption that useful info is rare. In general, facts will be complete and relatively specific, but may not apply in the modern age, and might have even been deliberately misleading.

  • Check DC: 18
  • Quality: Mediocre
    • Basic details, such as a creature’s type or a spell/item’s school of magic would be remembered. A few additional facts, such as one or two of a creature’s traits or the specific effects of a type of magic would also be known.
  • Near Fail: New Lead
    • If a character fails by 5 or less, they still recognize the topic and can usually identify an individual or other source to find out more. The information gained from such a source, however, will generally still be of mediocre quality (as above).

A topic that the character would have almost certainly have heard of before, if not in great detail. Such information could be incomplete (if it was literally overheard) or could be biased/misleading based on the source.

  • Check DC: 14
  • Quality: Decent
    • Basic details, as specified above, would be remembered. Additional facts, such as one or more of a creature’s traits, or the effects of a type of magic as well as its best defenses, would also be known.
  • Near Fail: Basics & Experts
    • If a character fails by 5 or less, they will still remember basic details. Additionally, they will also know a good place to get more information, and might even know a place to get even higher quality details than they themselves would have remembered.

A topic that the character would definitely have encountered before, both in great detail and with good frequency. Any information would be as correct as possible for the topic, and even if it included any sketchy or uncertain facts the character would easily point those out as being less reliable.

  • Check DC: 10
  • Quality: High
    • In addition to any information any lower tier would have access to, a character would also easily be able to comment on the trustworthiness and realism of any additional facts they learn or encounter later.
  • Near Fail: Research Start
    • If a character fails by 5 or less, they will still know all the most basic details on the subject. They will also know one or more high-quality sources to research more info from, and will still be able to semi-reliably comment on the veracity and quality of any new facts they learn on the topic later.

This is a topic that it is literally impossible for the character to have encountered before. Whether the topic is something that is brand new (such as a newly developed magical item or spell) or is something completely alien to common knowledge (such as an entity recently arrived from the Far Realm), there is no way a character could know this information. An optional tier of familiarity.

  • Rather than roll, a player should simply be told “your character has never heard of this, and it doesn’t sound even slightly familiar to them.”
  • If a player agitates for a roll, or has some other circumstance that might allow them one, use a DC of 20.
  • On a success of this roll, the character still knows no facts or details themselves, but can guess at where they might be able to get at least some information on the topic.

This is a topic that the character is a studied expert on; either it is an intrinsic part of their profession or daily practice, or it is a hobby or interest they have followed for many years. When a character fails a roll in the Familiar category by 5 or more and knows of someone they can ask for more details, that someone is in this category. An optional tier of familiarity.

  • Only rare or insanely specific facts would ever require a roll. This roll would have a very low DC, such as 5 or less.
  • Additionally, you might allow a character that falls under this category to roll a DC 18 Insight or knowledge skill check when learning any new fact about the topic. On a success, they can judge the truthfulness of the fact, identify if it might stem from misunderstanding, and speculate on why the fact hasn’t shown up in other records or sources until now.


EX: Illithid (Mind Flayers)

The following is an example of how a given topic would be detailed with the common knowledge system. Further topics will come in subsequent posts.

The illithid are a subterranean race of psionic brain-eaters who live in communities ruled by massive, disgusting Elder Brains. They kidnap other sentient life to feast on their brain matter, and to transform certain subjects into new mind flayers themselves.


Barring any specific encounters with mind flayers, the following groups likely have had no contact with them and would treat them as an almost wholly unknown topic.

  • Cultures: Humans, Elves (except drow), Halflings, Half-Elves, Dragonborn, Gnomes (except deep gnomes), Tieflings, Half-Orcs
  • Backgrounds: all save Hermit and Sage
  • Classes: all save Monks, Psions, or any class with psionic powers

While they have had no sustained contact with mind flayers, some aspect of the illithids remains within the myths, legends, and fairy tales of the following groups.

  • Cultures: Dwarves (except duergar)
  • Backgrounds: Hermits, Sages
  • Classes: Monks, any class with psionic powers (except Psions)

Due to repeated chances at encountering illithids, the following groups would have access to at least partial knowledge on the mind flayers.

  • Cultures: Drow, Deep Gnomes
  • Classes: Psions

With a long and oftentimes bitterly hate-filled history with the mind flayers, the following groups are almost universally familiar with the illithid race.

  • Cultures: Duergar, Githyanki, Githzerai
  • Backgrounds: N/A
  • Classes: any class with familiarity with the Underdark (Gloom Stalker Rangers, Circle of the Land Druids from the Underdark)



That’s the basics of the common knowledge system! As time goes by I’ll be putting up posts compiling different interrelated topics. Mind flayers are actually one of the easiest topics to define with this system, since pretty much every character can be sorted either into “if they knew about illithids they’d be brainless and dead by now” or “their entire culture revolves around the psychic trauma inflicted by generations of brutal illithid slavery”, which simplifies things quite a lot.

Other topics will be more difficult, but hey… that’s half the fun.

Let me know what you’d like to see next!

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